2001-10-20 04:00:00 PDT Portland -- Interior Secretary Gale Norton said yesterday that "we did make a mistake" in mischaracterizing patterns of rare caribou calving in reviewing how oil drilling could affect Alaska's Arctic National Wildlife Refuge.

The fate of the refuge has become one of the most contentious environmental issues facing Congress and the Bush administration, which strongly favors drilling and argues that it would reduce America's dependence on foreign oil supplies.

Norton's misstatement about the caribou occurred in a July letter to a Senate committee in which she downplayed the environmental impacts of drilling.

"We will take steps to clarify and correct that," Norton said of her error.

However, she addressed only one of a series of discrepancies between data provided to her by the U.S. Fish and Wildlife Service and the information she provided Congress.

In her July 11 letter to the Committee on Energy and Natural Resources, she was responding to its questions about how oil drilling could affect the Porcupine caribou herd that migrates annually to the refuge and its environs.

Documents released by an environmental advocacy group that represents government employees show that a review of caribou impacts by the Fish and Wildlife Service -- an agency Norton oversees -- was changed substantially in Norton's letter to Committee Chairman Frank Murkowski, R-Alaska. The Washington Post reported some of those changes yesterday.

In her remarks in Portland, Norton explained why she erroneously wrote Murkowski that in regard to the Porcupine caribou, "Concentrated calving occurred primarily outside the 1002 area in 11 of the past 18 years."

In fact, she said, the Fish and Wildlife Service had reported correctly that concentrated caribou calving did occur in the 1002 area -- a part of the refuge's coastal plain designated for drilling -- in 11 of the past 18 years.

"That was correct. It was correct on the maps," Norton said. "In sending that letter, we transposed it, saying that in 11 of the 18 years, that the calving appeared outside of the (proposed oil exploration) area."

Norton did not address other criticisms of her letter that mixed and matched different sources of information, except to say, "We also had information that came from other bureaus, and we incorporated that in the letter."

Leaders of Public Employees for Environmental Responsibility, which released draft documents from the Fish and Wildlife Service and the final letter, said Norton substantially altered the agency's biological findings.

Documents show that the Fish and Wildlife Service reported that calving reproduction of the Central Arctic caribou herd appears to have been affected by oil development in Prudhoe Bay. For instance, the birth rate for an undisturbed area was 83 percent in 1988-94, while the rate was 64 percent in a developed area in the same period, the service wrote.

Norton's letter states, "Parturition and recruitment data do not support the hypothesis that oil fields adversely affect caribou productivity."